In facilities where radiation emitting sources are used, for example, in hospitals where cancer patients receive radiation treatments or in blood banks where blood products are irradiated, various methods are used to quantitatively determine the radiation dose delivered by the source. The methods practiced include the use of thermoluminescent dosimeters (TLD's), ionization-type radiation detectors, photographic film, and radiochromic materials. TLD's are inconvenient because they require a complicated and time-consuming read-out process. Ionization-type radiation detectors are awkward and unwieldy and require a complicated setup. Photographic film requires a time-consuming chemical processing procedure before read-out. Radiochromic materials are preferred because they do not require any post-exposure processing and they are capable of measuring radiation doses with a high spatial resolution, but they are inconvenient in current practice because the calculation of the dose requires a complex sequence of steps, subject to error.
U.S. Pat. No. 5,637,876 describes a radiation dosimeter, exemplarily for use in determining a level of radiation to which a patient is subjected during radiation treatment, which comprises a substrate provided with a layer of radiation sensitive material. The radiation sensitive material has an optical density which varies systematically in accordance with the degree of radiation exposure. The dosimeter may take the form of a card or a flexible substrate which is positionable on the patient or other irradiation subject and which is also positionable in, or slidable through a slot in, a dose reader which includes a reflection or transmission densitometer.
The radiation sensitive material of a radiation dosimeter may be comprised of micro-crystalline pentacosadiynoic acid (PCDA) dispersed in a polymer matrix. Subjecting monomeric PCDA crystals, or related compounds such at the metal salts of PCDA, to ionizing radiation results in progressive polymerization, the degree of polymerization increasing with radiation dose. The amount of polymerization (and hence, the radiation dose) can be determined by measuring either the optical density or the spectral absorption of the exposed dosimeter. However, it has been found that these parameters also vary with both the temperature of the device when measured as well as the thickness of PCDA dispersion and the moisture content of the polymer matrix. Maximum accuracy of dose measurement must account for the temperature and thickness and moisture effects.
Radiation dosimetry film provides a means for measuring radiation exposure at a point, but its principal utility is in obtaining a two-dimensional map of radiation exposure, i.e. radiation exposure at multiple points in a two-dimensional array. A typical user may measure an 8″×10″ size film in one, or more, color channels at a spatial resolution of 75 dpi, generating a map of radiation doses at 450,000 points. Of course, other resolutions can be used to generate the radiation exposure map.
In practice, there is a problem presented by the measurement of the radiation sensitive film at a multiplicity of points. The problem is the availability and cost of means to make the measurements. Measurements of optical absorbance of the active component of a film (e.g. PCDA or the lithium salt of PCDA, LiPCDA) at the primary absorbance peak and other components at predetermined wavelengths would require the use of a scanning spectrophotometer. Such equipment is not readily available and would be of high cost. Furthermore the speed of operation would be slow because of the low intensity of the light source at the specific wavelengths where measurement is required.
A possible solution to the problem is to employ a film or document scanner to collect measurements of the film. The advantage of such means is that these scanners are widely available, they are of relatively low cost (often <$1000), they scan at high spatial resolution (up to 2400 dpi), they are rapid in operation (8″×10″ scan at 75 dpi resolution in <30 seconds), and they are adapted to measure color.
A film scanner is not like a spectrophotometer. It does not measure absorbance at specific wavelengths, but rather measures over a band of wavelengths. The band of wavelengths over which a specific model of scanner operates is defined by a combination of factors including the spectral output of a light source, the spectral absorbance of optical filters in the light path and the spectral response of the detector. A scanner adapted for color measurement typically will assess light absorbance integrated over three bands of wavelengths defining red, green and blue portions of the visible spectrum. The contribution of light absorbance at each wavelength to the total signal within a color band varies wavelength by wavelength. The weight at each wavelength is not user-defined, but rather depends on the aforesaid factors of spectral output of a light source, the spectral absorbance of optical filters in the light path and the spectral response of the detector.